Transparent blow-molded polyester containers are the de facto standard for carbonated soft drinks (CSD), and are increasingly being used for other pressurized beverages (e.g., juice and beer). The polyester most commonly used, polyethylene terephthalate (PET), can provide superior clarity, pressurized strength, recyclability and ease of manufacture, at a competitive price.
One of the most successful pressurized PET containers has a five-foot base design as described in Krishnakumar U.S. Pat. No. 4,785,949. The base includes a generally hemispherical bottom wall with five outwardly projecting feet symmetrically disposed about the circumference of the base. Following widespread adoption of the Krishnakumar 5-foot base, there have been numerous development efforts to modify this base, including use of elliptical (rather than generally hemispherical) bottom walls, adjustments to the size of the foot pad (on which the bottle rests), and modifications to the rib area between the feet (which can be susceptible to stress concentrations and stress cracking). For example, adjustments in the relative proportions of the feet and rib area may effect the pressure deformation (creep) of the container base, while adjustments in the depth and width of the feet may affect the ease of manufacturability (blow molding) of the feet. Two common problems are underformation of the feet, creating “rockers” which lack a uniform standing surface, and stress whitening, the creation of visual defects due to overstretching and/or cold stretching of the feet.
In these development efforts it is generally desirable to light weight the container base as much as possible in order to reduce the material cost. However, light-weighting has generally been accompanied by a reduction in base performance, such as increased creep and/or stress cracking.
Certain applications require a pressurized container incorporating one or more gas barrier materials, typically as a multilayer structure including one or more barrier layers alternating with one or more structural layers. However, pressurized multilayer containers often have a problem with layer delamination, either during pressurized filling, or later during storage and use.
Burst pressure is a commonly specified parameter which defines the minimum or average pressure at which containers can be filled without substantial evidence of cracking and/or delamination. Drop impact is another test for delamination and crack resistance. In one drop impact test a filled and capped container is dropped from a height of six feet onto a hard (e.g., steel) surface and then inspected for layer separation, crack formation, and potential crack failure (leakage). There are also simulated storage tests, at elevated temperatures and pressures, which provide an indication of performance under expected conditions of warehouse and/or retail storage and use.
Due to variations in thickness and orientation in different parts of the base, the base of a pressurized multilayer container is particularly susceptible to cracking, creep and delamination. For example, the central base region may be thickened and substantially amorphous, the ribs (between the feet) may be thinner and have a modest level of orientation, and the feet may be even thinner and have a relatively high level of orientation. The transition areas, where different levels of orientation and thickness meet, are areas of stress concentration and a likely source of base failure. In contrast, the substantially cylindrical and highly biaxially oriented sidewall of the container is less likely to fail.
It would be desirable to provide a pressurized plastic container having a multilayer footed base which provides pressure and delamination resistance, and in particular to provide a preform and corresponding container base design which can be light weighted and still provide pressure and delamination resistance.